1. Field of Invention
The present invention relates to piezoelectric transformers. More particularly, the present invention relates to a piezoelectric transformer having segmented electrodes on one or both faces of a piezoelectric ceramic disk. The transformer may be configured with a resonant feedback circuit that provides step up voltage transformation, and may provide voltage to multiple loads.
2. Description of the Prior Art
Wound-type electromagnetic transformers have been used for raising or lowering input voltages (step-up and step-down transformation, respectively) in internal power circuits of devices such as televisions or in charging devices of copier machines which require high voltage. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize high transformation ratios, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce.
To remedy this problem, piezoelectric transformers utilizing the piezoelectric effect have been provided in the prior art. In contrast to the general electromagnetic transformer, the piezoelectric ceramic transformer has a number of advantages. The size of a piezoelectric transformer can be made smaller than electromagnetic transformers of comparable transformation ratio. Piezoelectric transformers can be made nonflammable, and they produce no electromagnetically induced noise.
The ceramic body employed in prior piezoelectric transformers takes various forms and configurations, including rings, flat slabs and the like. A typical example of a prior piezoelectric transformer is illustrated in FIG. 1. This type of piezoelectric transformer is commonly referred to as a xe2x80x9cRosen-typexe2x80x9d piezoelectric transformer. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 to Rosen, and numerous variations of this basic apparatus are well known in the prior art as illustrated in FIGS. 2 and 3 which show disk-shaped and annular Rosen-type piezoelectric transformers, respectively. The typical Rosen-type piezoelectric transformer comprises a flat ceramic slab 110 which is appreciably longer than it is wide and substantially wider than thick. As shown in FIGS. 1 and 3, a piezoelectric body 110 is employed having some portions polarized differently from others. In the case of the prior art transformer illustrated in FIG. 1, the piezoelectric body 110 is in the form of a flat slab which is considerably wider than it is thick, and having greater length than width. A substantial portion of the slab 110 the portion 112 to the right of the center of the slab, is polarized longitudinally, whereas the remainder of the slab is polarized transversely to the plane of the face of the slab. In this case, the remainder of the slab is actually divided into two portions; one portion 114 being polarized transversely in one direction, and the remainder of the left half of the slab, the portion 116 also being polarized transversely but in the direction opposite to the direction of polarization in the portion 114. In the annular Rosen type transformer of FIG. 3, some portions of the annulus are polarized in the thickness direction, and the remaining portions are polarized in a circumference direction.
In order that electrical voltages may be related to mechanical stress in the slab 110, electrodes are provided. If desired, there may be a common electrode 118, shown as grounded. For the primary connection and for relating voltage at opposite faces of the transversely polarized portion 114 of the slab 110, there is an electrode 120 opposite the common electrode 118. For relating voltages to stress generated in the longitudinal direction of the slab 110, there is a secondary or high-voltage electrode 122 cooperating with the common electrode 118. The electrode 122 is shown as connected to a terminal 124 of an output load 126 grounded at its opposite end.
In the arrangement illustrated in FIG. 1, a voltage applied between the electrodes 118 and 120 is stepped up to a high voltage between the electrodes 118 and 122 for supplying the load 126 at a much higher voltage than that applied between the electrodes 118 and 120.
An inherent problem of such prior piezoelectric transformers is that they have relatively low power transmission capacity. This disadvantage of prior piezoelectric transformers relates to the fact that little or no mechanical advantage is realized between the driver portion of the device and the driven portion of the device. This inherently restricts the mechanical energy transmission capability of the device, which, in turn, inherently restricts the electrical power handling capacity of such devices. Additionally, because the piezoelectric voltage transmission function of Rosen-type piezoelectric transformers is accomplished by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and xxe2x80x2-yxe2x80x2 surface areas) of the piezoelectric member, which changes are of relatively low magnitude, the power handling capacity of prior circuits using such piezoelectric transformers is inherently low.
Because the typical prior piezoelectric transformer accomplishes the piezoelectric voltage transmission function by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and xxe2x80x2-yxe2x80x2 surface areas) of the piezoelectric member, it is generally necessary to alternatingly apply positive and negative voltages across opposing faces of the xe2x80x9cdriverxe2x80x9d portion of the member in order to xe2x80x9cpushxe2x80x9d and xe2x80x9cpullxe2x80x9d, respectively, the member into the desired shape. Prior electrical circuits which incorporate such prior piezoelectric transformers are relatively inefficient because the energy required during the first half-cycle of operation to xe2x80x9cpushxe2x80x9d the piezoelectric member into a first shape is largely lost (i.e. by generating heat) during the xe2x80x9cpullxe2x80x9d half-cycle of operation. This heat generation corresponds to a lowering of efficiency of the circuit, an increased fire hazard, and/or a reduction in component and circuit reliability.
Furthermore, in order to reduce the temperature of such heat generating circuits, the circuit components (typically including switching transistors and other components, as well as the transformer itself) are oversized, which reduces the number of applications in which the circuit can be utilized, and which also increases the cost/price of the circuit.
Another problem with prior piezoelectric transformers is, because the power transmission capacity of such prior piezoelectric transformers is low, it is necessary to combine several such transformers together into a multi-layer xe2x80x9cstackxe2x80x9d in order to achieve a greater power transmission capacity than would be achievable using one such prior transformer alone. This, of course, increases both the size and the manufacturing cost of the transformer; and the resulting power handling capacity of the xe2x80x9cstackxe2x80x9d is still limited to the arithmetic sum of the power handling capacity of the individual elements.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because individual ceramic elements must be polarized at least twice each, and the directions of the polarization must be different from each other.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because it is necessary to apply electrodes not only to the major faces of the ceramic element, but also to at least one of the minor faces of the ceramic element.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because, in order to electrically connect the transformer to an electric circuit, it is necessary to attach (i.e. by soldering or otherwise) electrical conductors (e.g. wires) to electrodes on the major faces of the ceramic element as well as on at least one minor face of the ceramic element.
Another problem with prior piezoelectric transformers is that the voltage output of the device is limited by the ability of the ceramic element to undergo deformation without cracking or structurally failing. It is therefore desirable to provide a piezoelectric transformer which is adapted to deform under high voltage conditions without damaging the ceramic element of the device.
It is another problem with prior transformers that they cannot withstand heat loads in excess of 600 degrees F., without sustaining damage.
It is another problem with prior transformers that they have low power utilization efficiencies, such as magnetic transformers which have an efficiency loss of up to 40-50%.
It is another problem with prior transformers that in order to handle certain ranges of frequencies, they must have a large size.
Another problem with prior transformers is that the magnetic core and coiled wire can generate magnetic fields that interfere with surrounding circuitry.
In addition to xe2x80x9cRosenxe2x80x9d type piezoelectric transformers, thickness mode multilayer piezoelectric transformers are known, as shown in FIG. 4. U.S. Pat. No. 5,834,882 to Bishop describes a multilayered, laminated, piezoelectric transformer which demonstrates the ability to convert a primary or input voltage Vin to a secondary or output voltage Vout through the application of voltage Vin to a first polarized piezoelectric ceramic wafer. The application of a voltage Vin to the first piezoelectric wafer generates an extensional stress in that wafer which is then mechanically transmitted to a second tightly adhered polarized piezoelectric ceramic wafer which undergoes a similar and proportional extensional stress, producing output voltage Vout.
A problem with these types of prior multilayer piezoelectric transformers is that they are difficult to manufacture because it is necessary to bond at least two ceramic layers together.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because of the use of adhesives to bond the ceramic layers and electrodes.
Another problem with prior piezoelectric transformers is that for a particular thickness of ceramic, their resonant frequencies are dependent upon the diameter of the ceramic layer.
The term piezoelectric transformer is here applied to an energy-transfer device employing the properties of a piezoelectric material to achieve the transformation of voltage or current or impedance. It is a primary object of the present invention to provide a piezoelectric transformer comprising a ceramic element exhibiting piezoelectric properties, which has electrodes bonded to both faces of the ceramic element. At least one face of the ceramic element has multiple electrodes bonded to it such that deformation of the ceramic element across one electrode segment results in corresponding deformation of the ceramic element in adjacent electrode segments.
It is another object of the present invention to provide a piezoelectric transformer of the character described in which application of a first voltage across a first electrode segment causes a first deformation of the ceramic element at that electrode segment.
It is another object of the present invention to provide a piezoelectric transformer of the character described in which such a first deformation causes a corresponding deformation of the adjacent sections of the ceramic element in the same direction (i.e. substantially parallel to the interface plane).
It is another object of the present invention to provide a piezoelectric transformer of the character described in which such a deformation of the adjacent piezoceramic element sections produces a second voltage across the electrode segments at the adjacent sections of the ceramic element.
It is another object of the present invention to provide a piezoelectric transformer of the character described which may be easily and inexpensively produced.
It is another object of the present invention to provide a piezoelectric transformer of the character described which is easy to manufacture because it is sufficient to polarize each ceramic element only once and in only one direction.
It is another object of the present invention to provide a piezoelectric transformer of the character described which is easy to manufacture because it is sufficient to apply electrodes only to the major faces of a ceramic element, and which does not require application of electrodes to minor faces of the ceramic element.
It is another object of the present invention to provide a piezoelectric transformer of the character described in which electrode segments on a single face of the piezoceramic element are electrically isolated from each other.
It is another object of the present invention to provide a piezoelectric transformer of the character described which electrically isolates the voltage and current at the input to the device from the voltage and current at the output of the device.
It is another object of the present invention to provide a piezoelectric transformer of the character described which is easy to manufacture and miniaturize, for example by using Micro Electronic Machining Systems (MEMS).
It is another object of the present invention to provide a piezoelectric transformer of the character described which is easy to connect or install in an electric circuit, because it is sufficient to attach (i.e. by soldering or otherwise) electrical conductors (e.g. wires) only to electrodes on the major faces of the ceramic element.
It is another object of the present invention to provide a piezoelectric transformer of the character described which is operable throughout a broad thermal range.
It is another object of the present invention to provide a piezoelectric transformer of the character described wherein application of a voltage across spaced apart input electrode segments creates vibration of the transformer in a tangential, radial and/or thickness mode.
It is another object of the present invention to provide a piezoelectric transformer of the character described wherein application of a voltage across spaced apart input electrode segments creates a standing compression wave in the ceramic element.
It is another object of the present invention to provide a piezoelectric transformer of the character described wherein such a standing compression wave in the ceramic element is at the resonant frequency of the ceramic element.
It is another object of the present invention to provide a piezoelectric transformer of the character described wherein sequential application of a voltage across input electrode segments creates a traveling compression wave in the ceramic element.
It is another object of the present invention to provide a piezoelectric transformer of the character described wherein such a traveling compression wave in the ceramic element creates output voltages whose phases are dependent on the phases of the input voltages.
It is another object of the present invention to provide a piezoelectric transformer of the character described with a traveling compression wave in the ceramic element that may be used in polyphase power applications.
It is another object of the present invention to provide a piezoelectric transformer of the character described wherein the output voltages may drive more than one load.
It is another object of the present invention to provide a piezoelectric transformer of the character described wherein the output voltages may be added in series to drive a single load.
It is another object of the present invention to provide a piezoelectric transformer of the character described wherein the output of one electrode segment may be used to provide a feedback voltage to the driving circuit of the piezoelectric transformer.
Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description thereof.